Hidden city

Over the next few hours Evans and a small group of archaeologists hacked through more landmine-strewn jungle and waded through swollen rivers and bogs to discover the ruins of five other previously unrecorded temples and evidence of ancient canals, dykes and roads, confirming data from revolutionary airborne laser-scanning technology called lidar.

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The discoveries matched years of archaeological ground research to reveal Mahendraparvata, a lost mediaeval city where people existed on a mist-shrouded mountain called Phnom Kulen 350 years before the building of the famous Angkor Wat temple complex in north-western Cambodia.

Subsequent searches have identified another two dozen hidden temple sites.

Dr Evans, director of the University of Sydney's archaeological research centre in Cambodia, said the ''eureka moment'' in the discovery came weeks earlier when the lidar data popped up on a computer screen.

''With this instrument - bang - all of a sudden we saw an immediate picture of an entire city that no one knew existed which is just remarkable,'' he said.

Heng Heap, a one-legged, chain-smoking former Khmer Rouge soldier, guided the expedition, hacking the way through undergrowth and skirting landmines in the area where he knows every significant outcrop, stream and valley.

Lost civilisation in Cambodia

SMH photographer Nick Moir travels to Cambodia to cover Sydney University Professor Damien Evans leading several teams of archaeologists to the discovery of an entire Ankor city in the Siem Reap region of Cambodia. Using new maps and LIDAR to discover previously unknown sites where previously only a few isolated temples were known to be. The holy plateau region known as Phnom Kulen was also one of the last holdouts of the Khmer Rouge and is still heavily landmined with many of the ex Khmer Rouge villagers bearing the scars of mine injuries. Photo: Nick Moir

Injured in three landmine explosions and wearing a prosthetic plastic leg, Heng Heap said he was surprised when the archaeologists, using GPS co-ordinates, pointed him straight to temple sites that were buried or hidden by jungle and that he never knew existed.

''I knew some things were there but not all of them,'' he said between puffs of a village-made cigarette.

Fairfax Media recorded the archaeologists pulling away undergrowth at several sites to find pedestals from collapsed temples that were probably looted centuries ago.

Guided by the GPS, they stumbled across piles of ancient bricks.

They found two temple sites where no carved rocks or ancient bricks could be found scattered nearby, indicating they have never been looted.

They also found a cave with historically significant carvings that was used by holy hermits who were common during the Angkor period.

The lidar, or light detection and ranging data, revealed hundreds of mysterious mounds several metres high across the mostly buried city.

One untested theory was that they were tombs where the dead were buried but there could be many explanations.

''We are still trying to work out what these things were,'' Evans said. ''There may be implications for society today … for example, we see from the imagery that the landscape was completely devoid of vegetation.

''One theory we are looking at is that the severe environmental impact of deforestation and the dependence on water management led to the demise of the civilisation … perhaps it became too successful to the point of becoming unmanageable.''

For centuries Phnom Kulen has remained a holy place where tens of thousands of pilgrims come each year to bath and perform spiritual rites.

Archaeological research of sculptured caves and river beds showed the area remained occupied throughout the Angkor period between the ninth and 16th centuries.

But the lidar technology has confirmed that Mahendraparvata was built on Phnom Kulen before Jayavarman II descended from the mountain to build another capital near where Angkor Wat now stands.

''This is where it all began, giving rise to the Angkor civilisation that everyone associates with Angkor Wat,'' Evans said.

Built over hundreds of square kilometres with a population of hundreds of thousands, possibly a million people, Angkor was the largest low-density pre-industrial civilisation on Earth, dominating south-east Asia for 600 years.

According to Chinese scholar Zhou Daguan, who recorded life in the then lowland capital between 1294 and 1307, Angkor rulers presided over slave-based civilisations where people went naked to the waist, wrapped only in cloth.

They lived in thriving, low-density cities with canals and villages and temples dedicated to a god.

According to ancient scriptures, a Brahmin priest anointed Jayavarman II a ''universal monarch'' in 802. But little is known about the city he presided over.

Phnom Kulen was covered by jungle for centuries until loggers moved into the area in 1990s after years of civil war.

The area was a former stronghold of the Khmer Rouge, a Maoist-influenced organisation that failed spectacularly in the 1970s to replicate the agricultural achievements of the Angkor period, causing the deaths of more than a million people from overwork or starvation, while hundreds of thousands more were executed.

Heng Heap's village, called Anlong Thom, is in the middle of the discovered city but none of the 1200 brooding villagers knew it.

David Sandilands, an Australian working in the village for the London-based Archaeological and Development Foundation, said more than half the villagers were malnourished.

''It is hard for the Khmers to think about history when they have empty stomachs,'' he said.

Sandilands is showing the villagers how to grow mushrooms that are nutritious and can be sold in markets.

He said the village was a ''fractured society'' but he hoped the villagers would benefit from intensified archaeological research on the mountain in that it would provide work and additional income. More than 25 villagers are already employed to clear vegetation and protect the mountain's historical sites.

When sites are being excavated in the 37,500-hectare Kulen National Park, the Archaeology and Development Foundation employs more than 100 people, mostly villagers.

But the work has been restricted by landmines that were laid indiscriminately across the mountain during the civil war.

Areas around the mountain's known historical sites are thought to have been cleared of the mines but they still pose a risk to archaeologists and villagers.

Now the lidar technology is set to replace the need for explorers and scientists to rely on the machete to clear the dense vegetation that covers the remains of mediaeval civilisations.

The technology was used in 2009 to reveal extensive terraced farming and a road network in the ancient Maya city of Caracol in the Central American country of Belize.

It has also been used recently at Stonehenge and other European archaeological sites.

The instrument fires rapid laser pulses at the landscape and a sensor mounted on it measures the amount of time for each pulse to bounce back.

By repeating the process, the instrument builds up a complex picture of the terrain it is measuring.

When Evans learnt of the technology he helped set up the Khmer Archaeology Lidar Consortium, made up of eight organisations including the foundation, Cambodia's APSARA National Authority and the University of Sydney's Robert Christie research centre.

The project was a gamble: the technology had never been used for archaeology research in tropical Asia. It would require the broadest co-operation ever among diverse groups of archaeologists from seven nations who were working in Cambodia and would cost more than a quarter of a million dollars.

High-level approval had to be sought from the government in Phnom Penh.

The consortium commissioned Indonesian company PT McElhanney to transport a Leica airborne laser scanner to Cambodia.